Some people show no symptoms. Others show an influenza-like illness with sudden onset of fever of more than 38.0^oC for more than 48 hours, headache, weakness, nausea, myalgia, backache, as well as, abdominal pain and photophobia. The symptoms last for 2 to 7 days after which the immune response to infection becomes detectable with the appearance of IgM and IgG and the patient recovers. While most human cases are relatively mild, a small proportion of patients develops a much more severe disease.